SPC - Collecting & Plotting Data

Data Collection

When planning to set up SPC on a process there are a few rules that need to be observed:

• Data source* - the data to be collected should be from one source only, in other words avoid collecting data from:
  • More than one process
  • More than one tool
  • More than one mould or casting cavity
  • More than one measuring station
• Data Type - data must be variable data (something that can be measured)
• Characteristic - should be restricted to those that are critical in terms of customer satisfaction, or identified as critical as part of the FMEA output.

* Date that is collected from more than one source will contain the variance between the two sources and will make it difficult (if not impossible!) to predict outcomes. It can even be misleading, in terms of interpretation of the completed SPC Chart. This will be fully explained in the next article 'Interpreting The Chart'

For the purpose of this exercise we will use some data that is from a manufacturing process, the diameter of a small shaft. The data is arranged from top to bottom, starting in the top left corner (1.071, 1.032, 1.164 etc).

The measurements are taken approximately every thirty minutes and five consecutive samples are taken each time

The shaft specification is 1.100 mm diameter + / - 0.010mm.

Plotting Data

We will now go through the process of filling in the data and calculating the various parameters to enable the results to be plotted, we will cover step by step. The plotting will take place in three areas as shown below:

Step 1: For each sub-group record the individual values then calculate the mean value of this sub-group.

Example:   (1.071+1.032+1.164+1.041+1.064) ÷ 5 = 5.372 ÷ 2 = 1.0744 ( X-bar).

Step 2: Calculate the range of this sub-group.

Example: 1.164 (largest Number) – 1.032 (smallest value) = 0.132

Note: When beginning a new SPC project it is best to run at least twenty sub-groups before attempting to plot the data. This will allow you to select the best fit for the graphs (more about this in the next article).

Step 3: Calculate the average of all the averages for X. This is the sum of all the averages divided by the number of sub- groups:

Step 4: Calculate the average for the ranges (R-bar). This is the sum of all the ranges for each sub group divided by the number of sub groups:

Step 5: Determine the scales for plotting:

• The range values
• The X-bar values

Step 6: Plot the range values and the X-bar values

Plot of the range values R

Plot of the average values X-bar

Step 7: Calculate and plot the control limits for range R and sub-group averages X-bar

There is a completed  example chart, using the data provided in this article, that can be downloaded (see below) that will enable you to follow this example.

This will also be referred to in the next article

Step 8: Plot the individual vales. To do this you will need to determine the intervals and the scale for plotting. Remember, the individuals should not be compared to the plot of X-bar as these are the average values of the sub-groups.

In the next article we will be discussing how to interpret an example of a completed chart.